The sharing of a communications resource based on a demand assignment multiple access allows this resource to be assigned to a user wishing to transmit data by means of this resource, without using it continuously. This technique allows a limited resource to be shared between many users accessing this resource at the same time. Such a resource is managed by an allocation plan assigning time slots to the users, according to their needs, so as to allow them to access the resource. When a user wishes to transmit data, he initially sends a request for capacity in order to reserve the communications resource needed for the transmission of the message. An allocation plan is generated periodically so as to take into account the requests for capacity from the various clients.
FIG. 1 shows an aeronautical communications system between aircraft 11.1, 11.2, 11.3, 11.4 and a gateway 12. The system uses a satellite communications resource, known as a return channel resource allowing messages to be transmitted from the aircraft 11.1, 11.2, 11.3, 11.4 to the gateway 12. Each of the aircraft 11.1, 11.2, 11.3, 11.4 comprises onboard applications and a terminal for communicating with the gateway 12. The messages sent by the aircraft are associated with services. The following may for example be mentioned: the service COTRAC (acronym for Common Trajectory Coordination) which allows the pilot and the air traffic controller to coordinate the trajectory of the aircraft in real time. A message from a service, which has a certain size, is associated with a priority and an end-to-end timing requirement of the communications network.
The satellite communications resource is based on a demand assignment multiple access. The communications system also comprises a control centre NCC 13 (NCC for Network Control Centre) responsible for sharing the satellite communications resource between the aircraft (whose number can be higher than several thousands).
The gateway 12 transmits the messages to a terrestrial network ATN connecting airline operational centres AOC (acronym for Airline Operational Control) and air traffic controllers connected to the air traffic management network via air navigation service providers ANSP (acronym for Air Navigation Service Provider).
In the framework of safety communications, in other words those requiring a high level of integrity and reliability but also having stringent timing requirements for some of them, the period of time required for the retransmission of lost messages or fragments of messages can lead to the non-compliance with these timing requirements. Indeed, the losses of packets are non-negligible over a return path aeronautical channel and these losses in the case of a demand assignment access lead to retransmission times incompatible with the timing requirements of the safety applications.
In aeronautical communications, the quantity of messages generated per user is low but the timing requirements are strict and a limited resource is shared by a large number of users. Indeed, it is necessary to receive 95% to 99% of the messages from a given service within a limited time (a 1.4 s period at 95% for the most demanding services in the control of air traffic). Moreover, a demand assignment multiple access (DAMA) leads to a significant additional delay over and above the time for detection of a loss of message (or fragment) due to the process of request/allocation for the data to be retransmitted. This additional delay may easily lead to the non-compliance with the timing requirements of the safety services and potentially to a non-optimum use of the satellite resource.
Furthermore, in the known technical solutions, the coupling of the mechanisms for detection and for retransmission of the data with the management of the resources is limited to an evaluation of the number of lost data values in a terminal in order to adapt the power, the modulation and/or the coding used by a terminal but does not affect the allocation of the resources in the case of a demand assignment multiple access of the DAMA type.
Other solutions allow the quantity of information to be retransmitted in the case of lost data to be reduced thanks to the use of optimized codes. These solutions can allow the number of errors over the channel to be limited or fewer resources to be used for the retransmission of the data, but they do not solve the problem of the delay induced by the necessary retransmissions which is a particular problem for safety communications, notably with regard to the communications for management of the air traffic which have very demanding time constraints for all of the messages of their service. However, these solutions could be used where necessary as a complement to the solution provided.
In the case of systems of the DVB-S2/RCS (for Digital Video Broadcasting-Satellite/Return Channel via Satellite) type with very low packet loss rates (in particular DVB-S2), where the channel is of a virtually error-free type, the mechanisms for error detection and for retransmission are not generally used over the satellite link and the data losses over the channel are generally recovered by the higher levels, where this is necessary (transport, applications levels). This solution is not optimum in the context of the safety communications because, in this context, the packet error rates are much higher and the problem of the retransmission time for the data is not solved by this type of solution, and it is even made worse given that the detection and the retransmission take place from end-to-end (potentially involving the non-compliance with the timing requirements of the safety services).
Generally speaking, broadband telecommunications systems are known. These systems provide a user supported data rate and the possibility of preempting a data value to be retransmitted over a resource already allocated. If the channel is virtually error-free, the retransmission is not taken into account by the system but is implemented from end to end (by the application or the transport level where necessary). This solution is not applicable to the context of safety communications because the traffic profile differs; indeed, the traffic considered in aeronautical communications is not a supported traffic (as it can be in telecommunications). Moreover, the retransmission time would be made worse by an end-to-end processing, as has been previously mentioned.
Another family of solutions consists in using a random access to the resource which allows the predetermination of the time for request/allocation of the resource for a demand assignment access (DAMA). This type of access will introduce a much higher packet error rate in comparison with the errors of the channel owing to the collisions between the messages sent by the various terminals, which will therefore lead to a greater number of retransmissions. The time for the transmissions or the retransmissions of lost data should theoretically be reduced due to the elimination of the request/allocation phase, however, this gain is only theoretical since the setting up of a management of the collisions is required, which involves effective access times that are much longer in the case of a high collision rate associated with a high load. Thus, the retransmissions will be much more frequent and the gain in terms of time, due to the elimination of the request/allocation phase, will not be sufficient to compensate for the effective time for access to the resource (imposed by the management of the collisions) nor for the delays associated with these retransmissions. This type of solution does not therefore allow for fully meeting the timing requirements of the safety services, in particular for the management of air traffic, since, even if each aircraft has a very low average traffic, a large number of aircraft must be able to share a limited resource, which necessarily leads to peaks in traffic (at least temporarily). In this case, a large number of collisions results from these peaks in traffic and random access does not allow the timing requirements of the messages to be met, in particular for long messages (for which the probability that a fragment undergoes a collision is higher).